The invention relates generally to movable support platforms, and more particularly to a movable support platform for use with a surface profiling system.
Rolling surface profiling apparatus have been used for some time to measure the profile or contour of a material surface, floor, road, etc. For example, U.S. Pat. No. 3,056,209 issued to Robert E. Oliver on Oct. 2, 1962, discloses a three-wheel, continuous recording, analog computation device that endeavors to accurately measure the contours of a surface with respect to a reference plane by the use of a xe2x80x9cfloatingxe2x80x9d center wheel which produces a vertical displacement signal with respect to the distance traveled over the surface by a double integration procedure. Equations 1 through 4 in the Oliver patent describe the geometrical and mathematical relationships between the measured and desired quantities that make such a device theoretically possible. However, the reference notes that it is necessary to consider stability problems and proceeds to change the ideal coefficients given in Equation 4 to certain values which the patentee found, through experience, to yield satisfactory results. The patentee justifies the abandonment of the ideal mathematical model as necessary to overcome serious problems of noise compounding that is inherent in all such integration devices.
A three-wheel rolling digital surface measurement apparatus is disclosed by Allen Face in U.S. Pat. No. 5,535,143. Similar to the Oliver patent, three collinear, sequentially oriented, regularly spaced and approximately equi-diameter wheels are provided on one side of a measurement platform. The middle wheel is a floating wheel having a linear position transducer coupled thereto. The rear wheel has an odometer coupled thereto. On-center spacing between each adjacent pair of wheels is given as S.
More specifically, the device of U.S. Pat. No. 5,535,143, illustrated schematically in FIG. 1, includes a rigid frame 2 rotatably supporting a rear wheel 3 and a front wheel 4 that are co-linear and separated by the distance 2S. Support wheels 3 and 4 contact the measured surface 1 at points i and i-2, respectively. Midway between support wheels 3 and 4, a sensing wheel 5 is in contact with surface 1 at point i-l. Sensing wheel 5 supports an axially movable column 6 that is connected to frame 2 in such a manner that its movement relative to frame 2 is restricted to an axis normal to the line joining the centers of wheels 3 and 4. A linear position transducer 7 having an output signal R is mounted on frame 2 in such a manner that its electrical output is directly proportional to the position of column 6 relative to frame 2. An odometer 8 is mounted on frame 2 to produce an electrical signal D that is directly proportional to the distance traveled by rear wheel 3 across surface 1. The linear position transducer signal R and odometer signal D are both input to a digital computer 9 which is programmed to interpret the two signals and record the instantaneous position, in convenient dimensions, of column 6 relative to frame 2 every time rear wheel 3 travels the distance S across surface 1. All of the above named individual components, as well as the electrical powering apparatus (not shown) required for the apparatus, are commercially available items. Similarly configured surface curvature measurement devices have long been known to those skilled in the art.
In accordance with the teachings of U.S. Pat. No. 5,535,143, each wheel will contact the measured surface at that point where the tangent to the wheel and the tangent to the surface coincide. Due to the undulation of the measured surface, the line connecting the center of each wheel and its associated contact point will rarely be perpendicular to the elevation datum. While the computer of this invention assumes that every reading point will fall on a normal from the wheel center to the elevation datum, in actuality, most of the reading points will be slightly displaced from the assumed position owing to the wheel surface contact geometry.
The computer of this invention records the column position transducer signal at the instant the odometer indicates that the rear wheel has traveled the distance S. However, between successive reading points, the rear wheel is not traveling in a straight line, but along an undulating surface. Thus, while the computer assumes that the horizontal displacement between successive readings is the constant distance S, in actuality the straight line distance between each successive reading will vary slightly according to the length of the undulating surface profile over which the real wheel travels.
The odometer triggering thus necessarily results in a slightly variable reading point spacing. Accordingly, the sensor wheel and front wheel contact points at one reading position will rarely coincide exactly with the rear wheel and sensor wheel contact points at the next reading position.
To be perfectly accurate, each measurement would have to contain an infinity of decimal places. Since the number of digits carried by the computer is finite, there is a rounding error introduced when the decimal infinity of the true measurement distance to the right of the last computer carried digit is dropped. Thus, the computer rounds the last carried digit up or down depending upon the value of the leftmost dropped digit. When the leftmost dropped digit is below 5, the computer rounds down thereby understating the true measurement and when the leftmost dropped digit is 5 or more, the computer rounds up thereby overstating the true measurement.
In addition to the reading point positioning and rounding problems described above, as a consequence of the physical imperfections inherent in the construction of the device, there will be a fixed characteristic error associated with each one of the finite number of possible column position transducer reading states. In addition to having a fixed characteristic error component, each column position transducer reading will also contain a completely random noise error. U.S. Pat. No. 5,535,143 goes on to describe a fairly complex mathematical relationship for dealing with these errors. However, there is another source of error not recognized, and therefore not addressed, by the any of the prior art surface profilers. Specifically, all surface profilers assume that the profiler""s wheels are the same diameter. However, since no two wheels are ever truly identical in size (due to manufacturing differences), one revolution of a profiler""s center (transducer) wheel will never cover the same distance as one revolution of its rear (odometer) wheel. Thus, even if the measurement platform is rolled in a straight line, measurement points will never be in exact coincidence as the measurement apparatus is rolled over a surface. Although this is a small incremental error, the effects of compounding can lead to inaccurate surface measurement over longer measurement runs. This is a substantial problem in the case of road surface profiling where a measurement run is on the order of a quarter mile or longer.
Accordingly, it is an object of the present invention to provide a movable platform that can be used as the measurement platform for a surface profiling system.
Another object of the present invention to provide a movable platform for use as part of a rolling digital surface profiling system.
Still another object of the present invention to provide a movable platform having regularly spaced and collinearly aligned wheels that guarantees alignment between subsequent surface contact points for each wheel revolution.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a movable platform has a frame for supporting a surface measurement system thereon. At least four supports are coupled to the frame and contact a surface so that the frame is supported above the surface. A first three of the supports are arranged in a linear alignment that defines a direction of travel for the frame, while a remainder of the supports are spaced apart from the linear alignment. The first three supports are defined by a front support, a rear support and a center support centered between the front support and rear support. The center support is a floating support capable of substantially vertical movement. At least two of the front support, rear support and center support are wheels configured to roll in the direction of travel defined by the three supports that are linearly aligned. Synchronization means are coupled to these wheels for synchronizing rolling movement thereof on the surface.